Suddenly I Feel More Comfortable With My Boilers Efficiency

blrman07 wrote:You have a lot of %'s flying around. What do they mean in the real world of dollars and cents? What good does it do to argue over whether or not you get 62% or 70% if there is nothing you can do about it?

Rev. Larry

In the end that wraps it up about as well as it can be wrapped up!

After doing all the math I concluded based on 75% efficiency that I use 19531 btus per hour to heat 2400 square foot house, domestic hot water and keep my outdoor hot tub in the mid 90's all winter long. Does that figure seem reasonable?

Lightning wrote:After doing all the math I concluded based on 75% efficiency that I use 19531 btus per hour to heat 2400 square foot house, domestic hot water and keep my outdoor hot tub in the mid 90's all winter long. Does that figure seem reasonable?

I'm currently projecting to be on target to use only about 14,725 output BTU's per hour on average for the 7 heating season months, to heat 2,100 sq-ft and provide DHW for 2, so based on your larger load demand I would say your average of 19,531 BTU's/Hr. seems quite reasonable. I also assume your locations annual HDD's are greater than mine.

2 years ago when it was so warm all winter here, I only needed an average of about 13,350 output BTU's per hour for the entire season.

Ok great. That's why I think you must do better than 62%. To me, It's NOT reasonable that my handicapped hand fed would get better efficiency than yours. I know you take good measurements that are hard to dispute. But, couldn't there be another explanation?

For the 10 years that we were all electric here, our annual KWH's of electricity ran roughly from a low of 24,000 KWH's (for a very mild winter) to a high of 30,000 KWH's (for a very cold winter). Averaging these extremes gives me 27,000 KWH's per year for 10 years. But that was to heat the home to 62 degrees when we were not home (we both work) and also at night, and 68 degrees during the day only when we were home. On top of this, the basement and bedroom zone loops were always kept at 62 degrees, and the living room, dining room, kitchen loop was the only one we bumped to 68 degrees when home. Now we maintain 69 degrees in the winter on all zones (sans the garage). We don't use much AC unless its really hot, and even when we run the AC the T-Stat is set at 75-80 degrees.

Take away heat and DHW from this since the fire up of the AHS Coal Gun and now we are currently using only around 6,500 KWH's of electricity per year for everything else that is still electric.

Ok. Let's isolate the variables. You keep the house warmer with coal? And your electric use. I assume your comparison doesn't include what the house uses. Right? You only counted heating kilowatts?

Lightning wrote:Ok. Let's isolate the variables. You keep the house warmer with coal? And your electric use. I assume your comparison doesn't include what the house uses. Right? You only counted heating kilowatts?

All of the homes KWH's were (and are) annually counted. The only changes are to our heat and DHW, which are now using coal. Plus the house is warmer in the winter now. 27,000 KWH's annually before and 6,500 KWH's annually now = 20,500 KWH's saved annually by using coal.

The past 2 years we averaged 4.6 tons of coal used for both heat and DHW during the 7 month heating season, and an additional 1 ton used for DHW only during the 5 non-heating months. 5.6 tons of coal per year overall. Electricity here is delivered at $0.133/KWH. My delivered and taxed coal cost is $243/ton.

Rob R. wrote:By the way, the biggest improvement I ever made as far as coal savings was a few cans of expanding foam to seal up drafts in the foundation...

Similarly, the biggest difference I noticed was piping combustion air from outside to the fan. The basement was not nearly as 'drafty' at the far end from the stove, I suppose because the combustion air requirement was not pulling air from the crawlspace under the kitchen.

Larry. Just to be clear. You are comparing the 20,500 kilowatts to the coal that's consumed. Right?

Lightning wrote:Larry. Just to be clear. You are comparing the 20,500 kilowatts to the coal that's consumed. Right?

Yes sir! But remember that my coal boilers efficiency during the non heating months is terrible. Figure about 300 KWH per month for providing our DHW from electricity (pre coal). Over 12 months that was 3,600 KWH. Take that as well as 6,500 away from 27,000 and you get about 17,000 KWH to heat our home for an average year using electricity. I figure we use about 4 to 4.5 lbs. of coal daily during the heating season strictly for DHW, and the rest of our coal usage is for heat during that time. For those 7 months we use 30.5 x 4.25 x 7 = ~900 lbs to heat the homes water.

Ok. So if you factor in that the house is warmer than with electric wouldn't it make your efficiency look better? Are you adding any space that wasn't counted before?

If our heating season coal usage is 4.6 tons of coal that is 9,200 lbs. of coal. Take 900 lbs away from that for DHW and it has required 8,300 lbs. of "heating" coal to displace 17,000 KWH of "heating electricity".

17,000 / 8,300 = 2.05 KWH/lb of coal

2.05 x 3,412 = 7,000 BTU's

7,000 BTU's/12,150 BTU's per pound = 57.6% efficiency for coal

But then add back in some unquantified efficiency guess factor to account for ~64 average degrees for the home with electricity vs. 69 average degrees with coal and that's where I've assumed 62.5% efficiency for the coal.

What it boils down to is that If the average 5 degree difference in home heat is worth more than about 5% of additional efficiency, then I'm doing better than 62.5% efficiency. I have only assumed about 1% saved for every 1 degree of T-Stat cut-back.

If I had alternately assumed 5% energy savings for every 1 degree of T-Stat set-back, for 5 degrees that would be 25%. Add that to 57.6% and you have an efficiency of 82.6%. Which is correct? Personally I can't imagine gaining 25% savings from setting the T-Stat back 5 degrees.

New twist. For the previous 2 heating seasons I have been burning about an average of 1.4 lbs. of coal per HDD (heating degree day). If I drop my homes temperature 5 degrees for the 215 days of heating, that is

5 degrees set-back x 215 heating season days = 1,075 HDD's saved per season due to a 5 degree T-Stat cut-back.

1,075 HDD's x 1.4 lbs./HDD= 1,500 lbs. of coal saved per heating season for a 5 degree T-Stat set-back.

8,300 lbs used the past 2 years on average minus 1,500 lbs = 6,800 lbs. to heat our home with a 5 degree T-Stat setback.

17,000 / 6,800 = 2.5 KWH/lb of coal

2.5 x 3,412 = 8,530 BTU's

8,530 BTU's/12,150 BTU's per pound = 70.2% efficiency for my coal boiler

It's also about 2.52% energy savings for every 1 degree of T-Stat set-back.

"It Depends" on your landed cost of a ton of coal = plus the labor of handling it cradle to grave....

but 5 degrees is a lot of house temp to drop to only save 3/4 ton of fuel material
--- unless your reducing the house temp from 78 to 73

Did the house humidity level also change?

Sting wrote:"It Depends" on your landed cost of a ton of coal = plus the labor of handling it cradle to grave....

but 5 degrees is a lot of house temp to drop to only save 3/4 ton of fuel material
--- unless your reducing the house temp from 78 to 73

Did the house humidity level also change?

Sorry, but I don't have a means to measure the humidity.